Examples of typical known systems for transmitting power between apparatuses arranged close to each other include magnetic-field-coupling power transmission systems in which power is transmitted from the primary coil of a power transmitting apparatus to the secondary coil of a power receiving apparatus utilizing a magnetic field. However, when power is transmitted through magnetic field coupling, since electromotive force is strongly influenced by the magnitude of magnetic flux passing through each coil, high accuracy is required in the relative positional relationship between the primary coil and the secondary coil. In addition, since coils are used, it is difficult to reduce the sizes of the apparatuses.
On the other hand, an electric-field-coupling wireless power transmission system is known, as disclosed in Japanese Unexamined Patent Application Publication No. 2009-296857. In this system, power is transmitted from the coupling electrodes of a power transmitting apparatus to the coupling electrodes of a power receiving apparatus through an electric field. This method allows the required accuracy of the relative positional relationship between the coupling electrodes of the power transmitting apparatus and the coupling electrodes of the power receiving apparatus to be relatively low and allows the sizes and thicknesses of the coupling electrodes to be reduced.
FIG. 1 is block diagram of a power transmission system 100 disclosed in Japanese Unexamined Patent Application Publication No. 2009-296857. This power transmission system 100 includes a power transmitting apparatus 152 and a power receiving apparatus 154. The power transmitting apparatus 152 includes a resonator unit 62 and power transmitting electrodes 64 and 66. The power receiving apparatus 154 includes power receiving electrodes 80 and 82, a resonator unit 184, a rectifier unit 86, a circuit load 88, a power measurement unit 120, and an impedance control unit 130. The power measurement unit 120 measures the level of power currently being supplied to the circuit load 88 by detecting a voltage across the two terminals of the circuit load 88, and outputs the measured power level to the impedance control unit 130. The impedance control unit 130, on the basis of the power level output from the power measurement unit 120, controls a voltage across a variable capacitance device Cv1 (for example, a varicap device) or the inductance of a variable inductance device Lv1, thereby maximizing the supplied power level.
In the power transmission system disclosed in Japanese Unexamined Patent Application Publication No. 2009-296857, the level of power currently supplied to the circuit load 88 is measured by detecting a voltage across the two terminals of the circuit load 88, and the frequency of an AC signal generated by an AC signal generator unit is controlled by controlling the capacitance of a variable capacitance device or the inductance of a variable inductance device, thereby maximizing the supplied power level. However, this control is complex.